Milling of metal powders in the presence of iodine



United States Patent John W. Dietz Wilmington, Delaware April 29, 1968 Nov. 17, 1970 E. I. du Pont de Nemours and Company Wilmington, Delaware a corporation of Delaware inventor App]. No. Filed Patented Assignee MILLING OF METAL POWDERS IN THE [56] References Cited UNITED STATES PATENTS 1,703,634 2/1929 Podszus 241/15 2,892,697 6/1959 Davieset al. 241/17 3,090,567 5/1963 Schafer 241/22 3,252,842 5/1966 Williams 241/16 3,301,494 l/1967 Tornqvist 241/22 Primary Examiner-Gerald A. Dost Attorney--Norbert F. Reinert ABSTRACT: iodine is employed as a grinding aid in the milling of metal powders. iodine [is particularly useful in milling of metal powders to submicrqn sizes.

MILLING OF METAL POWDERS IN THE PRESENCE OF IODIN E BACKGROUND OF THE INVENTION This invention relates to grinding aids for use in the milling of metal powders. Grinding aids are commonly used in the milling of metal powders to reduce reagglomeration of freshly milled particles, thereby reducing the milling time required and also lowering the minimum particle size that can be attained.

Organic acids, such as steric acid, and inorganic salts such as Na Cr O,{*7H O, K;,Fe(CN),; and (NI-I ),,M O **4H O are the most commonly used grinding aids. While these aids are relatively effective in preventing reagglomeration of very fine metal particles, they are difficult to remove once milling is complete. For example, if the organic acids are removed by heating, some decomposition will occur, leaving a residue of carbon and oxygen on the metal powder. Similarly, efforts to leach the inorganic salts from fine metal powders generally lead to oxidation of the metal.

This invention is founded on the discovery that iodine is superior to milling aids known to the art. More particularly, iodine is easy to handle; it is a solid at room temperature and can be weighed in the open air. More importantly, iodine is very effective in preventing reagglomeration offine metal particles, inhibits oxidation of fine metal powders and can easily be removed by heating under vacuum.

SUMMARY OF THE INVENTION In summary, this invention is directed to the following: In a process for reducing the average particle diameter of metal powders comprising milling said powder in the presence of a grinding aid, the improvement wherein said grinding aid is iodine. This invention is also directed to submicron powders obtained by this process.

DETAILED DESCRIPTION OF THE INVENTION Although the mechanism whereby iodine improves the milling of metal powders is not clearly understood, it is believed that the iodine adsorbs on and reacts with the freshly formed metal surfaces. The surface coating thus formed is sufficiently adherent to prevent the reestablishment of direct metal-to-metal bonds whenever two surfaces are pressed tightly together during subsequent milling. It is to be understood, however, that the scope of this invention is not to be defined or limited in any manner by this theory.

The mill, milling fluid and milling medium used in practicing the method of this invention should be relatively unreactive toward iodine. Steel is a suitable construction material for the mill. Suitable milling fluids and milling media are discussed hereinafter.

While the amount of iodine used in the method of this invention can be varied considerably, it is preferable to use an amount which is sufficient to provide at least a monatomic layer on all surfaces of the final milled powder. Lesser amounts of iodine are not as effective but still provide considerable improvement in milling efficiency. Larger amounts of iodine do not greatly improve milling efficiency. In fact, a gross excess of iodine is very undesirable. The excess iodine may react with the metal powder, forming metal iodides and resulting in a substantial yield loss. In addition, if the amount of iodine present is sufficient to react with substantially all of the metal powder, then the reaction may proceed so vigorously as to cause the mill to erupt. For these reasons, it is desirable to limit the amount to less than the amount which will react with ten percent of the metal powder.

For the above-mentioned reasons, from 0.001 to 0.05

grams, and preferably from 0.005 to 0,02 grams, of iodine per square meter of surface area in the final metal powder will ordinarily be utilized.

There is only a practical upper limit on the particle 'size of the starting metal powder; this is about 40mesh. The method of this invention can be used to mill larger particles, but it is ordinarily more convenient and economical to reduce the metal to minus 40-mesh, or somewhat less, by other methods known to the art. Preferably, minus 100- mesh powders are used as starting materials.

Iodine can be used as a grinding aid. for those metals, or mixtures of metals, which are conventionally reduced to very fine powders by milling, e.g., scandium, yttrium, lanthanum, titanium, zirconium, hafnium, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thuliurn, ytterbium, lutetium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, manganese, rhenium, iron, ruthenium, osmium, cobalt, rhodium, iridium, nickel, palladium, platinum, copper, silver, gold, aluminum, silicon and germanium,

Ballmilling is the conventionally preferred method of reducing the metal powders to submicron size. However, other milling procedures, such as air-attrition milling, can also be employed in practicing the method of this invention. The general rules of good milling practice should be followed in selecting the kind, size, and amount of milling media. A description of milling variables and the choice of optimum conditions is given in Chapter 16 of Micromeritics, J. M. DallaValle, Pitman Publishing Co., (New York 1934).

The milling media employed should be an abrasion-resistant material which has little or no reactivity toward iodine. Tung sten carbide bonded with cobalt is a preferred medium of this type. Another conventional milling medium is alumina. The milling media will ordinarily be spheres or cylinders whose diameters and lengths are equal. The size of milling media should be at least ten times the initial particle size of the powder to be milled, but should not be appreciably greater than half'an inch since this decreases milling efficiency. Optimum milling is generally obtained when the mill is filled approximately half full of media and the volume of powder and milling fluid is about equal to the volume of the voids in the milling media.

The mill should be completely enclosed or continuously purged with an inert gas so that moisture and oxygen cannot enter and react with the highly-reactive, fine metal powders. As is customary'in the art, it is ordinarily preferred to mill metal powders in an inert milling fluid, such as a light hydrocarbon oil. The oil helps protect the fine metal powder from oxidation, both during and after milling. The iodine coating also tends to protect the fine metal powder from oxidation. For this reason, it is often advantageous to leave the iodine coating on the metal powder even after milling is complete. In some instances, it will be most convenient to remove the iodine simultaneously with a subsequent processing step; for example, during the formation of sintered bodies from fine metal powders by heating in a vacuum or inert atmosphere, the iodine and metal iodide will sublime from the surface of the metal,leaving essentially no contamination in the product.

If it is'desired to obtain a pure metal powder prior to sub sequent processing, the iodine can be conveniently and completely .removed by heating. Heating under vacuum is preferred since this permits the removal of the iodine at a lower temperature when sintering and grain growth are less likely to 'occur. Depending on the quality of vacuum em ployed, and on the particular metal being treated, the temperaturerequired to effectively remove the iodine can vary from 20 to 600C. Generally, 100 to 300C. is most convenient.

The advantages of iodine as a grinding aid are particularly important when it is desired to produce metal powders having an average particle diameter of less than one micron. Such submicron metal powders are useful in the preparation of metal nitrides, carbides, phosphides, silicides, borides, etc. Incomplete reaction ordinarily results when coarser metal powders are utilized. Submicron metal powders are also used in mixtures with ceramics to form very strong composites or to average diameter ofless than one micron, as measured by conventional techniques for particle size counts on electron micrographs of fine powders. The preferred submicron powders of this invention are those having a surface area, by nitrogen adsorption, of at least one square meter per gram. The most preferred submicron powders of this invention are those having a surface area, by nitrogen adsorption, of at least four square meters per gram. The submicron powders of this invention are significantly less reactive toward atmospheric oxygen and nitrogen than are uneoated metal powders of the same surface area. The surface areas can be determined by the method described in A New Method for Measuring the Surface Area of'Finely Divided Materials and for Determining the Size of the Particles," by P. H. Emmett in symposium on New Methods for Particle Size Determination in the Suhsieve Range. at the Washington Spring Meeting ofASTM. March 4, 1941.

EXAMPLE] 180 Grams of minus l-mesh titanium powder, 12 grams of iodine and 9,500 grams of A-inch by A-inch cylinders of cobalt-bondedtungsten carbide are placed in a 2. lliter steel mill. Sufficient aliphatic hydrocarbon oil (b.p. l20l50C.) is added to just cover the mixture. The mill is then closed tightly and rolled at approximately 75 revolutions per minute for days. I

The resulting slurry is washed from the mill and grinding cylinders with additional oil. The slurry is allowed to settle and the clear, supernatant oil is decantedQThe residue is dried by heating to 100C. in a glass resin kettle under an absolute pressure of 0.05 mm.'of mercury for four hours. The sealed resin kettle is then transferred to a dry, nitrogen atmosphere where it is vented and the metal powder is removed. This metal powder is found to have a surface area, by nitrogen, adsorption, of 5.0 mI /g.

EXAMPLEZ I00 Grams of minus 100- mesh aluminum powder, 10 grams of iodine and 1,800 grams of alumina balls /&-inch in diameter are placed in a 2.l-1iter steel mill. The mill is purged with argon and closed tightly.' It is rolled at approximately 75 revolutions per minute for five days.

The mill is then transferred to a dry, nitrogen atmosphere where the product is separated from the alumina balls by screening through a l4 -mesh screen. The metal powder is found to have a surface area, by nitrogen adsorption, of 8 mi /g.

EXAMPLE 3 200 Grams of minus mesh copper, 18 grams of iodine and 3,000 grams of 'A-inch by /4-inch cylinders of cobaltbonded tungsten carbide areplaced in a l.3-liter steel mill.

Sufficient light aliphatic hydrocarbon oil (b.p. l30- 150C.)

is added to cover the mixture. The mill is then closed and rolled at approximately 75 revolutions per minute for five days. The slurry is washed from the mill and dried as in Example l. The resulting metal powder is found to have a surface area, by nitrogen adsorption, of 1.8 mF/g.

I claim:

1. ln a process for reducing the average particle diameter of a metal powder comprising milling said powder in the presence of a grinding aid, the'improvement wherein said grinding aid is iodine.

2. The process of claim 1 wherein the amount ofiodine utilized is sufficient to provide 0.00l to 0.05 grams per square meter of surface area ofthe product metal powder.

3. Theprocess of claim 2 wherein the starting metal powder is minus 40- mesh and the product metal powder has an average particle diameter ofless than one micron.

4. The process of claim 3 wherein said metal is titanium.

5. The process of claim 2 wherein the starting metal powder is minus lOO- mesh and the product metal powder has an avera e particle'diameter ofless than one micron.

6. e process of clalm 5 wherein said metal lS titanium. 

